1. Field of the Invention
The present invention relates to a foam-molded article using a polypropylene resin. More specifically, it relates to a polypropylene resin foam-molded article which is excellent in impact resistance, heat insulating property, stiffness and lightweight property and which is suitable for use in automobile parts, parts of electric appliances and heat/cold insulating containers.
2. Description of the Related Art
A polypropylene resin is a thermoplastic resin widely used for various molded articles due to its excellent mechanical properties and chemical resistance and cheapness. Efforts have been made to use a polypropylene resin in the field of foam-molded articles.
Methods generally used for making a foam-molded article of a polypropylene resin are a method in which a mold is charged with pre-foamed particles prepared by impregnating a polypropylene resin with a foaming agent and the particles are heated to fuse between them and secondarily foamed, thereby being molded into a desired shape and a method in which a polypropylene resin containing a foaming agent is heated, kneaded and extruded in the form of a sheet by means of an extruder to form a pre-foamed sheet, which is then subjected to hot press molding using a mold.
While the foam-molded articles obtained by these methods are excellent in heat insulating property and lightweight property, they are inferior in stiffness and mechanical strength and liable to be scratched on the surfaces.
JP-A 08-300391 discloses a method for increasing a mechanical strength of a foam-molded article, that is, a method in which a foaming material in a molten state is injected and filled into a cavity of a mold having a capacity of 10 to 95% based on a predetermined volume of a molded article and the cavity capacity is expanded up to the predetermined volume of the molded article to complete foaming while only the surface layer of the forming material contacting the mold is cooled to solidify and the inside thereof remains in a molten state, thereby providing a non-foamed layer on the surface of the molded article (hereinafter called a cavity expansion system foaming method).
Expansion of a cavity capacity in the middle of a molding step is achieved by sliding one side of a mold. Accordingly, when a box type molded article is to be obtained, a satisfactory foaming state is achieved at a bottom face part due to a large difference between the cavity thicknesses before and after sliding, but a non-foaming state is liable to be present at a side wall part.
Making use of such characteristics, JP-A 10-128795 proposes a method in which a non-foaming layer is provided on the surface of a main body part of a foam-molded article and a non-foamed rib part is provided.
However, because of a large difference in hardness between a main body part having a foamed layer and a non-foamed rib part, the foam-molded article obtained by the above method has the defect that crack or rupture is liable to be caused in a boundary between the main body part and the rib part when a force of impact or twist is applied.
In order to overcome this defect, it is conceivable that the inside of the rib part is allowed to be foamed by increasing a thickness of the rib part. This idea, however, is not advantageous in that the reduction in profitability caused by the increased weight is more significant than a reinforcing effect brought about by the resultant rib. For enhancing the reinforcing effect of a rib in molded articles of the same weight, it is more effective to provide a lot of ribs having a small thickness. Such a rib usually has a thickness of 3 mm or less, though in some specific cases exceeding 5 mm.
Further, increasing the amount of a forming agent to be added is one idea in order to allow the molded article to be foamed even inside the rib part. However, this is not practical because it may bring about bubbles of increased sizes and unevenness as well as a reduction in the number of independent bubbles, which will in turn result in a reduced strength and a poor appearance of the foam-molded article obtained.
An object of the present invention is to provide a polypropylene resin foam-molded article having a rib of a relatively small thickness in which the strength at a boundary part between a main body part and a rib part is improved without increasing a thickness of the rib part.
Intensive investigations repeated by the present inventors in order to solve the problems described above have resulted in finding that the object described above can be achieved by a polypropylene resin foam-molded article obtained by molding a material for a foam-molded article using a polypropylene resin, wherein the molded article comprises a main body part and a rib part; the rib part has a thickness (Tr) of 3 mm or less, or a thickness ratio (Tr/Tb) of a thickness of the rib part (Tr) to a thickness of the main body part (Tb) is 0.8 or less; the main body part and the rib part comprise a non-foamed surface layer and an intermediate layer having a foamed structure; and the intermediate layer in the main body part is connected to the intermediate layer in the rib part and that such a foam-molded article is obtained by molding a material for a foam-molded article using a polypropylene resin having specific MFR and melt tension. Thus, they have completed the present invention.
More specifically, the present invention is composed of the following aspects:
(1) A polypropylene resin foam-molded article obtained by molding a material for a foam-molded article using a polypropylene resin, wherein the molded article comprises a main body part and a rib part; the rib part has a thickness (Tr) of 3 mm or less, or a thickness ratio (Tr/Tb) of a thickness of the rib part (Tr) to a thickness of the main body part (Tb) is 0.8 or less; the main body part and the rib part each comprise a non-foamed surface layer and an intermediate layer having a foamed structure; and the intermediate layer in the main body part is connected to the intermediate layer in the rib part.
(2) The polypropylene resin foam-molded article as described in the above item (1), wherein the values of a melt flow rate (MFR) of the polypropylene resin and a melt tension (MS) thereof at 230xc2x0 C. satisfy the following equation (I):
log(MS) greater than 0.515xe2x88x920.702xc3x97log(MFR)xe2x80x83xe2x80x83(I) 
(3) The polypropylene resin foam-molded article as described in the above item (1) or (2), wherein the polypropylene resin is a polypropylene resin composition (A) comprising 0.01 to 5 parts by weight of an olefin (co)polymer (a) and 100 parts by weight of a propylene polymer (b); the olefin (co) polymer (a) has an intrinsic viscosity [xcex7]E of 15 to 100 dl/g; and the polypropylene resin composition (A) has an intrinsic viscosity [xcex7]T of 0.2 to 15 dl/g.
(4) The polypropylene resin foam-molded article as described in any of the above items (1) to (3), wherein the material for a foam-molded article is a mixture of the polypropylene resin composition (A) and other polypropylenes, the mixture comprising 5% by weight or more of the polypropylene resin composition (A).
(5) The polypropylene resin foam-molded article as described in any of the above items (1) to (4), wherein the material for a foam-molded article is molded by a cavity expansion system foaming method comprising a) an injection step, b) a primary cooling step, c) a foaming step and d) a secondary cooling step.